Transformer is an indispensable component of different products having an electric power conversion function, but a traditional transformer must have sufficient space for accommodating a leadframe and a ferrite core as well as a reserved winding space, and thus the traditional transformer usually has a relatively large volume. However, the present development trend of various electronic products emphasizes on high efficiency and small size. Obviously, a traditional transformer has a relative large volume while all other components of the electronic product keep reducing their size. Furthermore, the transformer is installed above a circuit board of the electronic product, and too much space above the circuit board remains unused, and the remained space under the circuit board becomes crowded, and thus the effect of heat dissipation drops. As a result, the output power must be lowered to avoid overheat, and traditional transformers have difficulties in spatial layout for a thin product. However, there are thin transformers. In traditional thin transformers, metallic foil wires are laid on a plurality of printed circuit boards (PCB), and the plurality of PCBs are stacked, so that the plurality of metallic foil wires are stacked like a winding in the transformer and connected with a ferrite core module to form a transformer. Prior art of this sort has been disclosed in R.O.C Pat. Publication No. 422400 entitled “Improved transformer” in which the transformer comprises a set of ferrite cores, a set of circuit boards and an insulating coil, and a plurality of spiral loops are laid on the set of circuit boards and stacked to form a plurality of circuit boards, such that the plurality of spiral loops constitute a primary winding of the transformer, and the insulating coil clamped in the set of circuit boards serves as a secondary winding, and the ferrite core module and the set of circuit boards are positioned to form a transformer. The structure of the patented technology can reduce the overall height of the transformer, but the cost of stacking a plurality of PCBs is higher than the structure of a traditional transformer. Most of the transformers formed by stacking a plurality of PCBs are usually used for DC/DC conversion only due to the restriction of safety regulations. Area must be increased to meet the requirements of the safety regulations, but an increased area is not favorable for the spatial allocation of the circuits. Therefore, the two aforementioned structures of the prior art transformers have drawbacks, and require improvements.